Anaesthesia is generally considered to have four main components: hypnosis (reversible state of unconsciousness), amnesia (repression of memory), analgesia (pain relief) and stable innervation of the autonomic nervous system. In practice, anaesthesia is achieved by combining several different anaesthetics and/or sedatives, wherein each of the components has specific effects on one or several of the above-mentioned components.
During anaesthesia, for adequate hypnosis, patients must be carefully and continuously monitored to achieve an appropriate balance between delivery of too high or too low concentrations of hypnotic agents. Delivery of too low doses of the hypnotic agent(s) may result in a patient being aware of what is happening during a procedure and possibly a later recall of the procedure, whereas too high doses of the hypnotic agent(s) may involve the risk of damage to the patient's central nervous system, or may result in sickness after the procedure or in a delayed convalescence.
Conventionally, so-called surrogate standard parameters such as blood pressure, heart frequency, perspiration and/or lacrimation have been employed to monitor the patient's state of anaesthesia during a procedure or surgery. These surrogate parameters do not provide a direct measure of depth of anaesthesia, as they do not reflect the main target organ of anaesthesia, namely the brain, and they are affected by other drugs and surgery. Electroencephalographic (EEG) signals have been employed to achieve a more specific monitoring of the state of the brain during anaesthesia, as the brain is the target organ of hypnosis. Specific parameters are extracted from the rather complex EEG signal to provide a quantitative measure of the hypnotic component of anaesthesia, in particular to distinguish between consciousness and unconsciousness. By employing several EEG parameters, the specific states of consciousness associated with the level of anaesthesia may be distinguished with a greater degree of reliability. Commercial EEG anaesthesia monitors of this type are available, e.g. under the name “BIS” from Aspect Medical Systems, Inc., USA and Covidien plc, Ireland, “Entropy Module” from GE Healthcare, USA, and “Narcotrend” from MHH, Germany. In these systems, “depth of anaesthesia” is generally quantified by means of a scalar indicator ranging between 0 and 100, with values between 0 and 20 corresponding to deep anaesthesia and values between 80 and 100 corresponding to consciousness/wakefulness.
In order to further enhance the reliability and accuracy of anaesthesia monitoring, there have been attempts to combine EEG parameters with standard parameters into a single indicator.
For instance, US 2007/0167694 A1 discloses a method and an apparatus for anaesthesia and sedation monitoring in which an index value representative of a condition of a patient may be computed by integrating EEG, pulse oxymetry, ECG and auditory evoked potential (AEP) signals.
European patent application EP 1 495 715 A1 describes a method and an apparatus for analgesia monitoring based on a mathematical index that combines three physiological parameters. These parameters may comprise blood pressure, cardiac excitation, ECG and EEG data, where EEG includes EMG data. EEG and EMG data may be analysed by means of spectral entropic quantities, and the mathematical index may be based on a fuzzy rule-based reasoning procedure.
International patent application WO 02/100267 discloses a method and a system for monitoring the depth of anaesthesia that is based on both EEG parameters and an AEP analysis, and may take into account certain patient data such as age, weight, height and gender.
However, these methods and systems generally suffer from the disadvantage that their algorithms rely on a fixed and predetermined number of input parameters in order to provide a reliable estimate of the state of anaesthesia. In an operating theatre, it often happens that data acquisition of one or more of these parameters becomes unreliable or fails completely. For instance, failure of EEG data acquisition is a common problem in conventional monitoring systems, and may lead to the monitoring apparatus being switched off automatically, or at least to inaccurate readings. The medical personnel may then be left without an accurate indication of the patient's hypnotic state. In stress or emergency situations that are common in hospitals, especially in an operating theatre, this may result in the patient receiving inadequate anaesthetic doses, with the detrimental consequences described above. Similar problems are encountered in the monitoring of vigilance.
Hence, what is needed is a method and a system for quantifying the depth of anaesthesia that provides a greater level of accuracy and reliability.